1. Field of Use
This invention relates generally to control systems for variable speed hydraulic motors such as are used in mobile self-propelled aerial lifts which have an adjustably movable work platform on which the operator's controls for the functions of the aerial lift are located.
2. Description of the Prior Art
Mobile self-propelled aerial lifts typically include a work platform which is supported at the outer end of an elevatable telescopic boom which is carried on a swingable boom support structure on the vehicle chassis. Variable speed hydraulic motors are used to propel and steer the vehicle, rotate the boom support structure, raise and lower the boom, and extend and retract the boom. Some prior art electrohydraulic control systems for operating the aerial lift comprise an engine-driven constant flow pump located on the boom support structure for supplying pressurized hydraulic fluid through proportional solenoid operated proportional flow control valves to operate the hydraulic motors. In such prior art control systems, each proportional solenoid for a control valve is operated by a proportional type of electric control which operates the proportional valve so as to meter fluid flow therethrough to the hydraulic motor being controlled to thereby regulate the speed of movement of the motor and thus, the speed at which the particular function is carried out. Typically, the operator's control panel is located on the adjustably movable work platform and includes several proportional electrical controls as there are functions to be controlled. Proportional solenoid operated valves and the proportional electrical controls therefor are relatively complex and expensive and undue redundancy adds to the cost of manufacturing and servicing. The following U.S. patents depict prior art aerial lifts: U.S. Pat. Nos. 3,212,604 and 3,937,340.